Enameled sheet steel



Oct. 18, 1960 D. J. BLlcKwEDE ETAL 2,956,906

ENAMELED SHEET STEEL Filed June 22. 1959 2 sheets-sheet 2 \C`mo/v ./ZMAX. "\-57E5L 5,455

MANGANESE .20% ra 50% INVENTORS Dona/J J /ic/fwea/e John n( Frame warMayer Unite 2,956,906 ENAMELED SHEET STEEL Donald I. Blickwede, John W.Frame, and Edward H. Mayer, Bethlehem, Pa., assignors to Bethlehem SteelCompany, a corporation of Pennsylvania Filed June 22, 1959, Ser. No.821,931

13 Claims. (Cl. 117129) This invention relates to enameled sheet steel,and more particularly to an extremely low carbon sheet steel which is tobe subsequently coated with vitreous enamel.

Among the principal diliiculties encountered in enameling sheet steel isthe formation of fish scale, boiling and blisters Fish scale, which isthe chipping oi of small pieces of enamel coating after the firing Acauses small craters or black specks to appear on the surface of theenamel, which are especially noticeable on direct-on White or colorcoatings.

Blisters, on the other hand, are manifest as pimples n the surface ofthe enamel. They are also caused by the evolution of gaseous products ofthe reaction between the steel and the frit. Since sheets or particleswhich are vitreous enameled must generally possess a smooth and unbrokensurface if they are to be found acceptable, it is a prerequisite thatenameled articles be substantially free from fish scales and blisters.

A further requisite for sheet steel which is to be subsequently enameledis that said sheets must exhibit satisfactory sag resistance, i.e.resistance to warping or sagging during the tiring of the enamel.

Conventional enameling iron, i.e. steel having a carbon content of:O2-.03% and a manganese content of .G7-.08%, has substantial freedomfrom ish scale and blisters and also exhibits fairly good sagresistance.

States Patent However, there are certain processing disadvantages due lto the composition of conventional enameling iron, these disadvantagesbeing substantially reduced or completely w eliminated by reason of thecomposition of the superior enameling steel which is the subject of thepresent invention.

One of the ditliculties in processing conventional enameling iron occursduring the rolling of the metal into sheets and strip. Steel ofcompositions in which the total carbon and manganese content is lessthan .10%, such as conventional enameling iron, often exhibit a hotshort temperature range between l650 and l900 F., since there isinsufficient manganese to effectively combine with the sulfur in themetal. As hot rolling in this range may produce deep cracks on the edgeof the prodl uct, it is the usual practice to complete the roughing loperations above this range, allow the steel to cool through this range,and then to resume rolling below the hotshort range. It is obvious thatthis cessation during hot rolling is undesirable, as close temperaturecontrol and '2,956,906 Patented Oct. 18,- 19,60

2 tinuity in the entire operation thus reduces the tonnage output of themill.

Steel that has been subjected to cold reduction, e.g. sheet or strip, isinherently hard and fairly brittle because of internal strains producedby this cold reduction. It is therefore necessary to heat treat thesteel in order to impart the desired mechanical properties thereto priort0 subsequent forming into finished products. The two general methods ofheat treatment are annealing and normalizing, both of whichsatisfactorily relieve the strains from the metal. Conventionalenameling iron, because of a low manganese content of .D7-.08%, does notrespond to ordinary annealing temperatures, e.g. 1200- l300 F., butrather must be normalized at temperatures approximating l700 F. This lowmanganese content is thought to be necessary for good sag resistance,and it indeed appears to be necessary for steels of the generalcomposition of conventional enameling iron containing .O2-.03% carbon.Our` steel, on the other hand, by Virtue of Iits higher Mn content, canbe annealed at l200-1300 F. with consequent advantages in the simplicityof the softening heat treatment. At the same time it has good sagresistance in spite of its high Mn content because its carbon content isso low.

Enameling iron is widely used in applications Wherein deep drawingprecedes the enameling operation. AIt is therefore desirable to producean enameling iron which is both soft and ductile. Although conventionalenameling iron possesses these qualities, we believe our new steel to begenerally superior for deep drawing applications.

lt is a principal object of our invention to provide a new product whichnot only satisfies the aforementioned requisites for steel to beenemaled, but is actually superior to conventional enameling iron insatisfying these conditions.

It is a further object of our invention to provide ja new method ofmaking a vitreous enameled sheet steel.

Other objects will become apparent from the following description takenin connection with the drawings, in which f Fig. l is a graph showingsag versus temperature;1

Fig. 2 is a plan view of a vitreous enameled sheet embodying ourinvention, and

Fig. 3 is a view in transverse section of the sheet shown in Fig. 2. t f4 Compositions which have been found to have exceptionally goodqualities in achieving the objects of the present invention as above setforth come within the following approximate ranges: Y

Percent YManganese .2O-F50.

-' additional equipment are necessary, and this lack of con-V Sulfur.030 max. H Phosphorus .01 max. Remainder Essentially iron.

Although the sulfur and phosphorus contents mentioned are excellent forsteel sheet which is to be deep drawn prior to thev enameling operation,other ranges areof course permissible, the ranges mentioned being forthe purposes of example only, and in no way intended to limit the scopeof the invention. Y

It will become apparent later in the specification that although thecarbon range is given as .010% max., a lower carbon content results inan even betterproduct,

Y and the preferred embodiment of the invention is aY steel having amaximum carbon content of approximately .O03%.

One method of attaining these low carbon'levels comprises decarburizingsheet steel of e.g. .0o-.08% carbon by heating to a temperature of 1200to,1300 F. for an vitreous enameled in the usual manner.

Steels of these compositions are free from fish scale, boiling defects,and blisters upon enameling. Furthermore, as will later be pointed out`in greater detail in connection with the drawing, these steels have atleast as good sag resistance as conventional enameling iron up to l550F., and considerably better sag resistance between 1550 and 1600 F.Because of the composition there is no hot-shortness during the rollingoperation, and our steels can also be annealed at ordinary annealingtemperatures.

It is well known that primary boiling defects in enameled articles arecaused by the evolution of carbonaceous gases that are the products ofthe reaction between the iron oxide scale on the steel surface and thecarbon in the steel. During the enameling operation these gases bubblethrough the molten frit and either carry with them bits of scale orleave a crater on the surface. Both result in poor appearance,especially in direct-on white or color coats. By maintaining a maxi` mumcarbon content of .003% in the preferred embodiment of the invention,blisters are greatly minimized, and excellent results have been obtainedin single coat enameling, as well as in applications when more than onecoating is desired. It is also well known that fish scaling andreboiling are caused by the evolution of hydrogen from the steel. Thehydrogen is formed at the enameling temperature during the tiring, andis dissolved in the steel at that temperature. At room temperature it isless soluble in the steel and hence evolves, causing fish scales orblisters. Or it may be evolved on reheating for the second coat firingin which case it is called reboiling. For some reason, perhaps the lowersolubility of hydrogen in our very low carbon steel, the hydrogendissolved in the material as a result of the ring is not as great as inordinary enameling irons, and hence fish scaling does not occur evenwith direct-on white coating on both sides of the steel.

The enamel coating applied to the steel may be fired at hightemperatures, e.g. between 1550 and 1600 F. In ring at suchtemperatures, we have found, for example, that 20 gauge steel embodyingour invention (specifically having a carbon content of 003%, manganese.20% to .50%, and the remainder essentially iron) has excellent sagresistance.

Fig. 1 shows a comparison of the sag resistance of ordinary sheet steel,therein designated by solid line C, conventional enameling iron,designated by broken line A, and compositions comprising the preferredembodiment of the present invention, designated by solid line B. Allthese sheets were of the same gauge, in this case 20, although othergauges will, of course, have similar characteristics. The ordinary sheetsteel is of the same composition as steels of the present inventionexcept for carbon, i.e. having a manganese content of .20 to .50% and acarbon content of .06 to .08%. The conventional enameling iron has amanganese content of .07 to .08% and .02 to .03% carbon, while thepreferred steel of this invention has .20 to .50% manganese and carbonof the order of .003% max. While it is obvious that both enameling ironand steel of this invention are far superior to ordinary sheet steel, itis readily apparent that our new compositions have better sag resistancethan enameling iron in the temperature range 1550 .to

,4 1600 F., our steel showing an increase in sag of less than .10 in. ina 10 inch span, as measured in the center of said span between thesetemperature limits, while conventional enameling iron shows a sagincrease of more than .60 in. in a 10 inch span in this same temperaturerange. The total sag of the enameling iron at 1600" F. can be seen to bemore than twice as great as that of steels comprising the preferredembodiment of the present invention, while the sag of both theseproducts is approximately the same between 1400 and 1550 F.

rPbis sag apparently occurs principally at temperatures where the steelconsists essentially of ferrite and austenite, there being very littletendency to sag at lower temperatures where the steel consistsessentially of ferrite. Since, as has been heretofore mentioned but willbe discussed in greater detail later in the specication, it is highlydesirable for the steel to have a manganese content of .20 to .50%, thecarbon content of our steel is of prime importance. Due to the muchlower carbon content of compositions of the present invention, aconsiderably higher manganese content causes no decrease in sagresistance, since the higher transformation temperature associated witha lower carbon content offsets the lowering of the transformationtemperature caused by increasing the manganese, and our steel thus hasbetter sag resistance than conventional enameling iron between 1550and1600 F.

The high manganese content of our new steel is one of the salientfeatures of the invention, as the aforementioned diiculties in theprocessing of conventional enameling iron are substantially reduced orcompletely eliminated by the presence of .20 to .50% manganese in thesteel. Enameling iron, with a carbon content of .02 to .03%, isnecessarily limited to a manganese content of .G7-.08% to insure goodsag resistance, whereas the effects of higher manganese are offset bythe much lower carbon in our steel. A carbon content of :O2-.03% thereinis not low enough to allow a manganese content substantially greaterthan .G7-.08% and still impart good sag resistance.

There is no trouble with hot-shortness during the hot rollingoperations, as there is suicient manganese present in the steel toefficiently combine with the sulfur to form manganese sulde, auon-embrittling compound, thus preventing the formation of ferroussulfide, an embrittling compound. It is therefore unnecessary to interrupt the hot rolling of the steel to allow the metal t0 cool through thetemperature range in which hot shortness occurred in prior enamelingirons. Furthermore, our steel can be annealed at ordinary annealingtemperatures of 1200-1300 F., thus eliminating the additional costincurred in the normalizing necessary for conventional enameling iron,such normalizing being done at about 1700 F.

It has been found that after box annealing at ordinary temperatures of1200-1300 F. our steel 'has slightly coarser grains, the sheet issofter, more ductile, and generally superior to enameling iron for deepdrawing applications.

Although we have described our operation in considerable detail, we donot wish to be limited to the exact compositions shown and described butmay use such substitutions, modiiications or equivalents thereof as areembraced within the scope of our invention or as pointed out in theclaims.

We claim:

1. A vitreous enameled sheet formed from steel containing carbon .010%max., manganese .20 to .50%, sulfur .030% max., phosphorus .01% max.,and the remainder being iessentially iron.

2. A vitreous enameled article formed of steel containing carbon .010%max., manganese .20 to .50%, and the remainder being essentially iron.

y3. A vitreous enameled steel sheet containing carbon .010% max.,manganese .20 to .50%, and the remainder being essentially iron.

4. A vitreous enameled steel sheet containing carbon .010% max.,manganese .20 to .50%, sulfur .030% max., phosphorus .010% max., and theremainder being essentially iron.

5. A vitreous enameled `article formed of steel containing carbon .003%max., manganese .20 to .50%, and the remainder being essentially iron.

6. A vitreous enameled steel sheet containing carbon .003% max.,manganese .20 to .50%, and the remainder being essentially iron.

7. A vitreous enameled steel sheet containing carbon .003% max.,manganese .20 to .50%, sulfur .030% max., phosphorus .01% max.,remainder essentially iron.

8. An enameled sheet formed from steel containing carbon .003% max.,manganese .20 to .50%, the remainder essentially iron, said sheet duringthe enameling operation having a maximum difference of sag at 20 gaugethickness of .10 inch in a 10 inch span between 1550 and 1600 F.

9. An enameled sheet formed from steel containing carbon .003% max.,manganese .20 to .50%, the remainder essentially iron, said sheet havingthe property at 20 gauge thickness of sagging less than .40 in. in a 10inch span at a temperature of 1600 F.

10. A method of making a vitreous enameled article comprising applyingan enameling material to an article formed of steel containing carbon.01% max., manganese 6 .20% to .50%, and the remainder being essentiallyiron, and firing the coated article.

11. A method of making a vitreous enameled article comprising `applyingan enameling material to an article formed of steel containing carbon.003% max., manganese .20% to .50%, the remainder being essentiallyiron, and firing the coated article.

12. A method of making a vitreous enameled steel sheet comprisingapplying an enameling material to a steel sheet containing carbon .003%max., manganese .20% to .50%, sulfur .030% max., phosphorus .01% max.,remainder essentially iron, and firing the coated sheet.

13. A method of making a vitreous enameled steel sheet comprisingapplying an enameling material to a steel sheet containing carbon .003%max., manganese .20% to .50%, the remainder essentially iron, and tiringthe coated sheet, said sheet having the property at 20 gauge thicknessof sagging less than .40 inch in a 10 inch span at a temperature of 1600F.

References Cited in the le of this patent UNITED STATES PATENTS1,496,505 Whittaker et al. June 3, 1924 1,996,568 Butts Apr. 2, 19352,109,271 Krause IFeb. 22, 1938 2,455,331 Eckel et al. Nov. 30, 1948UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.2,956,906 october le, 1

Donald J Blickwede et al.,

It is hereby certified that error appears in the printed specificati ofthe above numbered patent requiring correction and that the said LettePatent should read as corrected below.

Column l, line 38, for "particles" read articles column 2, line 34, for"enemaled" read enameled Signed and sealed thi 25th day of April l9l(SEAL) Attest:

ERNEST W, SWIDER DAVID L LADD Attesting Ofcer i Commissioner of Pate]

2. A VITREOUS ENAMELED ARTICLE FORMED OF STEEL CONTAINING CARBON .010%MAX., MANGANESE .20 TO .50%, AND THE REMAINDER BEING ESSENTIALLY IRON.